The present invention relates generally to “wafer-level chip scale package” (WCSP) devices in which integrated circuit chips are not surrounded by opaque packaging material, and more particularly to avoiding photon-induced currents to flow in the chip's substrate as a result of surfaces of the chips, especially the edges thereof, being exposed to light.
A WCSP device, also referred to herein as a WCSP chip, is sometimes exposed to light because the silicon chip is not surrounded by any opaque packaging material (such as opaque plastic or epoxy encapsulation material), and light impinging on the chip causes photon-induced currents to flow in the chip substrate. The photon-induced currents can undesirably increase the magnitudes of input bias currents of circuitry in the WCSP chip. (Those skilled in the art know that an ideal amplifier has infinitely high input impedance and zero input bias current. Therefore, reduced input bias current is desirable in many applications, for example to achieve low input offset voltages and/or to reduce output errors in feedback amplifiers.)
FIG. 1 shows a conventional WCSP chip 1 having solder bumps 3 on the active surface of the chip. As shown in FIG. 1, WCSP chip 1 is inverted, and solder bumps 3 are physically and electrically connected to various conductors 4 on a surface 5A of a mounting substrate 5, such as a printed circuit board. WCSP chip 1 includes a silicon substrate 2 having a back surface 2A with a light barrier coating 6 thereon. Arrow 7 indicates the general location of the “active region” 12 (see FIG. 2) of chip 1 within which integrated circuitry is formed. No packaging or encapsulation is provided directly around WCSP chip 1 to prevent ambient light from impinging upon it.
FIG. 2 illustrates ambient light rays impinging upon WCSP chip 1. Some of the ambient light 10A is reflected from light barrier coating 6, and has no effect on the operation of the circuitry in active region 12. However, other ambient rays such as 10C-1 impinge upon the exposed edges of WCSP chip 1 and enter silicon substrate 2, as indicated by reference numeral 10C-2. Other ambient light rays such as 10B strike the surface 5A of mounting substrate 5 and are reflected one or more times between surface 5A and the bottom surface of the inverted WCSP chip 1 as it appears in FIG. 2 and eventually strike the chip surface in area 7 wherein the active region 12 is located. FIG. 3 illustrates photon-induced currents caused by the ambient light striking exposed areas of chip 2.
Referring to FIG. 3, photon-induced currents 13 are caused by ambient light 10C, some of which is indicated as being reflected from a surface 5A of the mounting substrate near the exposed surfaces of chip 2. Nearly all of photon-induced current 13 is collected by circuitry at the edge of active region 12 of WCSP chip 2. For example, FIG. 4 illustrates an amplifier 15 included in circuitry 12A in active region 12 of FIG. 3. Amplifier 15 has two external inputs which are connected to corresponding solder bumps 3-1 and 3-2, respectively. Circuitry 12A also includes two input protection diodes 21 and 22 connected to the two input terminals that are connected to solder bumps 3-1 and 3-2, respectively. Reference numerals 13A and 13B designate large portions of the photon-induced current 13 in FIG. 3 being collected by input protection diodes 21 and 22, respectively. The total bias current flowing into terminal 3-1 is equal to the sum of the actual input bias current Iinput of the (−) input of amplifier 15 plus the photon current Ip collected by input protection diode 21. Similarly, the total bias current flowing into terminal 3-2 is equal to the sum of an actual input bias current Iinput of the (+) input of amplifier 15 plus a photon current Ip collected by input protection diode 22. Therefore, if the photon-induced currents Ip are very large, the total input bias current of amplifier 15, i.e., the total input bias current flowing through input solder bumps 3-1 and 3-2, which also function as the input terminals of amplifier 15, also will be very large. For example, if chip 20 is not exposed to any light, the total input bias current flowing into input terminals 3-1 and 3-2 may be typically about 2 picoamperes. However, if chip 20 then is exposed to a typical amount of ambient room light, then the total input bias current flowing into input terminals 3-1 and 3-2 may be typically about 150 picoamperes.
Barrier rings of various types are widely used in integrated circuit devices and even in discrete transistors to collect various kinds of substrate currents to prevent such currents from degrading circuit performance or to prevent undesired circuit latching or other parasitic effects. For example, commonly owned U.S. Pat. No. 5,767,538 issued Jun. 16, 1988 to Mullins et al. discloses a doped barrier ring for preventing photon-induced currents caused by light impinging on an integrated photodiode included within an integrated circuit from reaching other circuitry included in the integrated circuit. However, use of barrier regions or rings can substantially increase the amount of chip area required for an integrated circuit without accomplishing a benefit that is worth the cost.
Thus, there is an unmet need for a device structure for a WCSP chip that avoids problems due to photon-induced substrate currents being collected by active circuitry in the chip.
There also is an unmet need for a WCSP chip device structure that prevents photon-induced substrate currents from substantially increasing the total input bias currents of active circuitry in the chip.